Field Spell Cards must be placed in the Field Zone. Since March 21st 2014 for the OCG and July 10th 2014 for the TCG, both players will be able to control a single Field Spell Card. Before, it was only possible to have a single Field Spell Card active on either side of the field, and when another one was activated, the previous Field Spell Card would have been destroyed via Game Mechanics.

You may activate a Field Spell Card at any time during your Main Phase; any card that currently occupies that Field Zone must first be sent to the Graveyard in order to activate a new one. You can also choose to Set a Field Spell Card. Setting a Field Spell Card does not start a Chain. Activating a Field Spell Card (from your hand or by flipping it face-up) does start a Chain. A Field Spell needs to remain on the field to resolve, so their effect(s) are not applied if they are destroyed before they can resolve (for example, if "Mystical Space Typhoon" is chained to that Field Spell Card's activation).